Re: [PATCH] aoe: adjust ref of head for compound page tails

[Ed Cashin <ecashin@coraid.com>]

On Aug 7, 2013, at 4:58 PM, Andrew Morton wrote:

> On Thu, 1 Aug 2013 21:29:59 -0400 Ed Cashin <ecashin@coraid.com> wrote:
> 
>> As discussed previously,
> 
> I think I missed that.
> 
>> the fact that some users of the block
>> layer provide bios that point to pages with a zero _count means
>> that it is not OK for the network layer to do a put_page on the
>> skb frags during an skb_linearize, so the aoe driver gets a
>> reference to pages in bios and puts the reference before ending
>> the bio.  And because it cannot use get_page on a page with a
>> zero _count, it manipulates the value directly.
> 
> Eh?  What code is putting count==0 pages into bios?  That sounds very
> weird and broken.

I thought so in 2007 but couldn't solicit a clear "this is wrong" consensus from the discussion.

  http://article.gmane.org/gmane.linux.kernel/499197
  https://lkml.org/lkml/2007/1/19/56
  https://lkml.org/lkml/2006/12/18/230

We were seeing zero-count pages in bios from XFS, but Christoph Hellwig pointed out that kmalloced pages can also come from ext3 when it's doing log recovery, and they'll have zero page counts.

I don't have a counter in the driver to track how often zero-count pages show up in bios, so I don't know how common it is these days.  I would have been motivated to look if I had gotten the impression that there was any interest in identifying and eliminating cases where zero-count pages were being used in bios.

-- 
  Ed Cashin
  ecashin@coraid.com